In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of flour to form a roux and cook for 1-2 minutes until lightly golden.

Gradually pour in 2 cups of milk while whisking continuously to avoid lumps. Stir until the sauce thickens, around 5-7 minutes. Stir in a pinch of nutmeg and set aside.

In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add 3 cloves of minced garlic and sauté until fragrant, about 30 seconds.

Toss in 8 ounces of sliced mushrooms and cook until golden brown, roughly 5-6 minutes. Stir in 4 cups of fresh spinach until wilted, about 2 minutes. Remove from heat.

Preheat your oven to 375°F (190°C). In a 9x13 inch baking dish, spread a thin layer of béchamel sauce across the bottom. Layer 3-4 lasagna noodles over the sauce, followed by half of the mushroom and spinach mixture. Sprinkle 1 cup of mozzarella on top.

Repeat layers finishing with noodles, béchamel, and ½ cup of mozzarella on top.

Cover with aluminum foil and bake in the preheated oven for 30 minutes. Remove foil and bake for an additional 15 minutes until bubbling and golden.

Let it cool for 10 minutes before slicing. Serve warm and enjoy!